An ultrasonic diagnostic apparatus is designed to generate ultrasonic pulses from transducer elements disposed in an ultrasonic probe. The ultrasonic pulses are emitted into an object to be examined. The ultrasonic diagnostic apparatus receives reflected ultrasound produced due to differences in acoustic impedance among the tissues of the object. The ultrasonic diagnostic apparatus displays image data on a monitor, which is generated based on reception signals corresponding to the reflected ultrasound. This diagnostic method allows easy observation of real-time two-dimensional images by simple operation of only bringing the ultrasonic probe into contact with the body surface. The diagnostic method is widely used for functional diagnosis or morphological diagnosis of various organs in a living body.
An Ultrasonic diagnostic method, which obtains living body information by using reflected ultrasound from tissue or blood cells in the living body, have rapidly progressed along with two great technical developments of an ultrasonic reflection method and an ultrasonic Doppler method. And B-mode images and colored Doppler images obtained by these techniques have become indispensable to recent ultrasonic image diagnosis.
Nowadays, an electronic-scanning ultrasonic diagnostic apparatus most widely used has a plurality of transducer element arranged in one dimension and displays two-dimensional image data in real time by controlling the driving of the transducer elements at high speed.
In a color Doppler method for generating colored Doppler image data, moving objects such as blood corpuscles in predetermined cross section of a living body is scanned by the ultrasonic pulse and Doppler frequency shift of the reflected ultrasound corresponding to blood flow velocity is detected and imaged. The colored Doppler method was applied in the past to generate image data for intracardiac blood flow with high velocity, but has become applicable to generate image data for blood flow with low velocity such as blood flow in abdominal organs.
By the way, high measurement accuracy (low flow velocity detection capability and high flow velocity detection capability), temporal resolution, and spatial resolution are required for obtaining high diagnostic accuracy in the colored Doppler method. When ultrasonic pulses are emitted to moving objects and moving velocity of the objects is detected from the Doppler frequency shift of the reflected ultrasound, conventionally, the transmission and reception of the ultrasonic pulses with respect to the objects is repeatedly performed a plural number of times (L times) with a predetermined transmission interval Tr and the moving velocity is measured on the basis of a series of reflected ultrasound obtained in the observation time Tobs (Tobs=Tr·L).
In this case, the detection capability of low flow velocity (low flow velocity detection capability: lower limit of measurable flow velocity) Vmin is determined by a characteristic of a filter (for example, MTI filter) used to detect Doppler components from a series of reflected ultrasound obtained through the L-times ultrasonic transmission and reception, that is, a cut-off frequency and a shoulder characteristic of the filter. At this time, Vmin is expressed by Expression 1 when it is assumed that repetition frequency of the transmission (rate frequency) is fr (fr=1/Tr).
                                          V            ⁢                                                  ⁢            min                    ∝                      1            Tobs                          =                  fr          L                                    (        1        )            
On the other hand, the upper limit of measurable flow velocity (high flow velocity detection capability) Vmax is determined by nyquist frequency defined as a half of repetition frequency of the ultrasonic transmission (rate frequency) fr and is expressed by Expression 2.
                              V          ⁢                                          ⁢          max                =                              C            ·            fr                                4            ⁢            fo            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            ξ                                              (        2        )            
Here, C denotes sound velocity in an object, fo denotes central frequency of reception ultrasound, and ξ denotes an angle formed by a transmission direction of ultrasounds and a blood flow direction. When the Doppler frequency shift is greater than nyquist frequency, accurate measurement of blood flow velocity is not possible because of the aliasing occurred in a frequency spectrum of a Doppler signal.
That is, in order to improve the low flow velocity detection capability Vmin which is a first requirement of the colored Doppler method, it is necessary to set the rate frequency fr low or to increase the number of transmission performed in a predetermined direction. In order to improve the high flow velocity detection capability Vmax, it is necessary to set the rate frequency fr high. However, when the rate frequency fr is set high, a next ultrasound is emitted before reflected ultrasound from a deep portion is not received. Accordingly, there occurs a problem of a residual echo that reflected ultrasound corresponding to a preceded rate period are mixed and received.
The real time property which is a second requirement is determined by the number of images displayed per unit time (frame frequency) Fn. And the frame frequency Fn is expressed by Expression 3. Here, N is the total number of transmission/reception directions necessary for generating a colored Doppler image data. In order to improve the real time property, it is necessary to set the number of transmission/reception times L or the total number of transmission/reception directions N small.
                    Fn        =                              fr                          L              ·              N                                =                                    1                              Tobs                ·                N                                      ∝                                          V                ⁢                                                                  ⁢                min                            N                                                          (        3        )            
Moreover, in order to improve the spatial resolution which is a third requirement, it is necessary to increase the number of transmission/reception directions N. That is, the frame frequency Fn, the low flow velocity detection capability Vmin, the high flow velocity detection capability Vmax, and the spatial resolution have trade-off relations each other, and thus it is difficult to satisfy these requirements simultaneously. Therefore, the frame frequency Fn and the high flow velocity detection capability Vmax are important for the blood flow measurement in a circulatory organ, and the frame frequency Fn and the low flow velocity detection capability Vmin are important for the blood flow measurement in an abdomen or a peripheral organ.
In order to solve such a problem described above, a so-called simultaneous and parallel reception method for increasing the number of receiving signal per unit time by transmission ultrasound to a predetermined direction and simultaneously receiving reflected ultrasound in a plurality of directions adjacent to the predetermined direction is performed.
However, when the central axis of a transmitted ultrasonic beam and the central axis of a received ultrasonic beam are different in simultaneous and parallel reception method, the transmission/reception sensitivity is deteriorated. Moreover, when the number of reception directions for the simultaneous and parallel reception is three or more, it is difficult to obtain uniform transmission/reception sensitivity in a lateral direction (a direction perpendicular to the transmission/reception direction of the ultrasounds).
In order to solve such a problem, a method of reducing the number of transducer elements used for ultrasonic transmission or a method of enlarging the beam width of transmitting ultrasound by weighting the amplitudes of the driving signals supplied to the transducer elements in a array direction thereof is disclosed in, for example, Japanese Patent Publication (Kokai) No.3-155843.
On the other hand, a method of simultaneously performing transmission and reception of ultrasound in a plurality of directions is disclosed in U.S. Pat. No. 5,856,955. In this method, when the transmission ultrasound are transmission in a predetermined direction by driving a plurality of transducer elements with driving signals having predetermined delay time, the transmission of ultrasound in a plurality of directions is simultaneously performed by combining the driving signals having the delay time corresponding to a plurality of transmission directions and supplying the combined driving signal to the transducer elements.
In U.S. Pat. No. 5,856,955, the non-uniformity in transmission/reception sensitivity is improved by enlarging the beam width of the acoustic transmission field in comparison with the conventional case. However, as the focused transmitting ultrasound is used, it is difficult to enlarge the beam width so as to correspond to an acoustic reception field. Accordingly, distortion of the transmission and reception fields (hereinafter mentioned as the “beam distortion”) or non-uniformity in transmission/reception sensitivity still remains.
FIG. 14 shows the beam distortion which is a first problem of the conventional method or the method disclosed in U.S. Pat. No. 5,856,955, and FIG. 15 shows the non-uniformity in transmission/reception sensitivity which is a second problem of the above-mentioned method.
FIG. 14A shows the simultaneous and parallel reception on the basis of an transmission field (solid line) and a plurality of acoustic reception fields (dotted lines) formed to overlap the acoustic transmission field when the transmission of ultrasounds is performed in a predetermined direction (central axis direction of the acoustic transmission field) by using a ultrasonic probe for convex-scanning in which the transducer elements are one-dimensionally arranged on a convex surface. In the figure, only the acoustic reception fields Br-1 and Br-3 corresponding to the sides of the acoustic transmission field Bt and the acoustic reception field Br-2 corresponding to the center of the acoustic transmission field Bt are shown for the purpose of simple description.
In the conventional simultaneous and parallel reception method, the transmission ultrasounds are focused at a predetermined position (depth) of the object, similarly to a non-simultaneous and parallel reception method, and ultrasonic energy is concentrated on the position. On the other hand, the reception ultrasound can form acoustic reception field continuously focused by applying a so-called dynamic focus method of sequentially moving focal point in the depth direction correspond to the reception time.
In this case, the transmission/reception sensitivity of ultrasound is determined by the product of an acoustic transmission field and an acoustic reception field (that is, an acoustic transmission/reception field). In the acoustic transmission/reception field formed by the acoustic transmission field Bt shown in FIG. 14A and the acoustic reception field (for example, the acoustic reception field Br-1) located at the side of the acoustic transmission field Bt, the acoustic transmission field in the focal area greatly affects the acoustic transmission/reception field. As a result, a beam distortion occurs in the central direction of the acoustic transmission/reception field as shown in FIG. 14B, and thus image distortion occurs in ultrasonic image data generated by the acoustic transmission/reception field Bo-1 or the acoustic transmission/reception field Bo-3 (not shown) having the beam distortion.
FIG. 15A schematically shows an acoustic transmission field pattern, an acoustic reception field pattern and an acoustic transmission/reception field pattern in the lateral direction of the simultaneous and parallel reception, where sound pressure at the side portion of the acoustic transmission field is smaller than that of the center. Accordingly, when the number of simultaneous reception directions is set to 3 or more, the amplitude of the acoustic transmission/reception field pattern (that is, transmission/reception sensitivity) is non-uniform in the lateral direction and dark and bright stripes are generated in the ultrasonic image data generated by the non-uniform transmission/reception field and deteriorates the image quality. The remarkable decrease in transmission/reception sensitivity at the side portion of the acoustic transmission field makes it difficult to estimate flow velocity, variance, etc in the colored Doppler image data, as well as image quality deterioration in the B mode image data.
When the acoustic transmission pattern is enlarged in the lateral direction as shown in FIG. 14B for the purpose of improvement in the non-uniform transmission/reception sensitivity, ultrasonic transmission energy is uselessly irradiated to areas not associated with generation of image data, thereby transmission/reception sensitivity is deteriorated and generation of a virtual images (artifacts) due to side lobes and multiple reflections increase.
That is, the beam distortion, the non-uniformity in transmission/reception sensitivity, and the decrease in transmission/reception sensitivity deteriorate the image quality of the ultrasonic image data and deteriorate diagnostic accuracy thereof.
On the other hand, when the acoustic transmission field for simultaneous and parallel reception is formed using the method disclosed in U.S. Pat. No. 5,856,955, it is necessary to combine a plurality of driving signals having delay times corresponding to a plurality of transmission directions, thereby causing a problem that the circuit configuration of a transmission unit is much complicated.